(a) Draw a stepwise mechanism for the reaction of ethyl...

Name: Problem 60, Chapter 24: Organic Chemistry
Uploaded: 2019-06-10T05:40:17-08:00
Duration: 2 min 15 s
Channel: Colton K
Description: (a) Draw a stepwise mechanism for the reaction of ethyl 2,4-hexadienoate with diethyl oxalate in the presence of base. (b) How does your mechanism explain why a new carbon-carbon bond forms on C6? (c) Why is this reaction an example of a crossed Claisen reaction?

(a) Draw a stepwise mechanism for the reaction of ethyl 2,4-hexadienoate with diethyl oxalate in the presence of base. (b) How does your mechanism explain why a new carbon-carbon bond forms on C6? (c) Why is this reaction an example of a crossed Claisen reaction?

Key Concepts

Nucleophilic Attack on the Carbonyl Group

Once the enolate is formed, it attacks an electrophilic carbonyl group in the other ester molecule. This step involves the formation of a tetrahedral intermediate, which is common in nucleophilic addition reactions. The subsequent collapse of this intermediate, with the elimination of a leaving group, leads to the formation of a new carbon–carbon bond.

Resonance Delocalization in Conjugated Systems

In a conjugated system, the delocalization of electron density over multiple atoms results in nucleophilic sites that can be located remotely from the original deprotonation site. This delocalization explains why a new carbon–carbon bond might form at a position (such as C6) that is not immediately adjacent to the carbonyl group, as the negative charge is spread over the conjugated framework.

Enolate Formation

In the presence of a base, an acidic hydrogen adjacent to a carbonyl is abstracted to form an enolate ion. This intermediate is resonance?stabilized and serves as a powerful nucleophile capable of attacking electrophilic carbonyl carbons, setting the stage for carbon–carbon bond formation in condensations such as the Claisen reaction.

Crossed Claisen Condensation

A Crossed Claisen condensation involves the coupling of two different carbonyl compounds, usually esters, under basic conditions. In this reaction, the enolate from one ester attacks the carbonyl carbon of a different ester, rather than another identical molecule. This differentiates it from a standard Claisen condensation and explains the formation of unsymmetrical products.

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